The Fluid Catalytic Cracking process unit (“FCC unit”) is one of the most important upgrading processes in a refinery. In the FCC unit, solid particulate catalyst makes intimate contact with vaporized or gaseous hydrocarbons in a column, forming a fluidized bed having temperatures controlled to effect the desired cracking of the larger hydrocarbon molecules.
The physical properties of the catalysts used in FCC units, “FCC Catalysts” are designed such that even though the catalyst is a solid, it behaves like a liquid when enough fluidizing media (i.e. vapors, air) are present. In general, the average particle size of a FCC catalyst particle is between 60 and 90 microns. Two important parameters are the percent by weight of the total below 20 microns and the percent by weight of the total below 40 microns. The first parameter is important as an indication of losses or potential losses, as the cyclonic systems used in FCC units are generally designed with a cutoff in efficiency at around 20 microns, and particles below 20 microns are more difficult to prevent from releasing to the atmosphere and therefore more likely to present an environmental problem. The losses can be detrimental both economically and environmentally. Cyclonic systems or cyclones and air separators are well known devices used to separate materials of different physical properties. In particular, cyclones can separate mixtures of solids with different particle size, density, relative mass or any combination thereof. Anything below 20 microns is lost very rapidly from the air cyclone process and is either captured by higher efficiency systems downstream or emitted to the atmosphere. The 0-40 micron content is important as particles in the 20-40 micron regime are important for fluidization in the system. Poor fluidization stemming from a relatively low component of smaller particles can result in reduced catalyst circulation or fluid bed instability.
Another particular characteristic of the FCC process is that in order to maintain the proper level of catalyst and the correct activity and selectivity of the system, a certain percentage of catalyst needs to be added to the FCC unit in a more or less continuous fashion. In general, catalyst addition rates (CAR, usually in tons per day or lbs per barrel) are added in the range from 0.05 to 0.5 lbs of catalyst per barrel of oil processed. Another way to characterize this is by saying that the CAR varies from ˜0.5% to ˜5% of the total inventory. In general these values are a consequence of many parameters that are related to both the unit and the oil feed properties. Because of this continuous (at least in principle) catalyst addition, catalyst losses and catalyst withdrawals, the actual catalyst working at any given time in an FCC unit is intrinsically a blend of catalysts with different ages. Each unit has its own characteristics and for those versed in the Art, the term Equilibrum Catalyst or ECAT represents the physical blend of the resulting catalyst that was formed in the FCC unit by controlling additions around a set of “Equilibrium Properties” that make the operation stable within an operating window which may or may not vary in time. This will become an important aspect of this invention as the “aging” process results in changes in the physical, chemical and catalytic properties of the original catalyst. Thus “equilibrium catalyst” or ECAT is by definition a used catalyst and contains at least some catalyst added to the original charge more or less continuously over the tenure of its use. Thus an “ECAT additive” as used herein is a material added periodically during the tenure of use of the ECAT. Equilibrium catalyst also often includes a significant portion of fines which are collected by the user over time, in a third stage separator, an electrostatic collector or other fines capturing device. The user will have added these collected fines to the ECAT before sending the ECAT to the processor. I intend for the definition of equilibrium catalyst to include ECATs that include such recycled fines. The term “recycled fines” includes such collected fines added to or mixed with an ECAT.
The chemical nature of FCC catalysts is well known to those versed in the Art. These catalysts generally contain zeolite Y in one or several forms (RE-Y, USY, RE-USY, CREY, etc) as the main source of activity and selectivity. Some other zeolites like ZSM-5 can be added to change the selectivities both as an intrinsic part of the catalyst or as a separate additive particle. Aluminas or Silica Aluminas of different properties are sometimes added to increase the activity for conversion of heavy molecules (bottoms upgrading). Clay is used as a filler that assists with the catalyst's chemical and physical properties. A binder, generally a low molecular weight oligomer of Silica (Silica Sol), or Aluminum Chlohydrol (Alumina Sol) is added to the mixture which is spray dried to form the particles that are shipped for usage, with or without a further post-treatment to adjust the chemical composition and set the desired selectivities. Another type of commercially available FCC catalysts, the so called “In-Situ” technology, takes preformed spray-dried particles of Kaolin clay and other materials like Silica and aluminas and chemically treats them to form Zeolite Y based FCC catalysts.
It is accepted by experts in the field that FCC catalysts suffer both hydrothermal and metal deactivation. The steam sources that cause hydrothermal deactivation are from both the process and consequence of combustion. The metals that cause catalyst deactivation originate in the feedstock and accumulate in the catalyst. In general, steam and high temperatures result in destruction of the zeolite. This effect is accelerated by the presence of vanadium especially when it is present in its V+5 state. These phenomena also result in a noticeable loss of surface area on the matrix. Thus, catalysts used with lighter feeds which usually contain less contaminant metals (Ni, V, Fe) are deactivated less severely than those in which the metals in the feed are substantially higher, thus keeping a larger fraction of the original activity and selectivity. CAR is usually higher for units with high metals in the feedstock.
As a catalyst particle ages within an FCC unit, it tends to pick up metals such as vanadium and nickel and to lose its crystallinity, both effects inversely increasing density even though the dimensions of the particle may remain constant. Density of the FCC ECAT is typically measured in terms of Apparent Bulk Density (ABD), usually in grams per cc. Iron poisoning is known to have changed catalyst morphology, resulting also in an increase in density. Because the efficiency of cyclones is dependant on the momentum (e.g. mass) of the particles, large (diameter), heavy particles (those having high ABD) have a much better retention than smaller ones. As a consequence, large, old particles are primarily removed from the FCC unit via catalyst withdrawals as their retention due to cyclone efficiencies increases as a function of time.
It is generally accepted that nickel is deposited on the outside shell of FCC catalyst particles and it is more or less immobile from particle to particle. However, although vanadium is also deposited in the outside of the particles, it is well established that it migrates intraparticle and from particle to particle. Conditions of high oxidation state and steam accelerate vanadium mobility and thus zeolite destruction. Nickel does not deactivate the catalyst and some even propose that it somewhat delays the activity of Vanadium. Nickel's most important effect is that it is a dehydrogenation catalyst which increases undesirable reactions resulting in high hydrogen and coke yields (V also has dehydrogenation activity. An accepted correlation is that V is ˜¼ as active as Ni for dehydrogenation). At constant particle size, Ni on a particle is thus a good measure of its relative age. Depending on the mobility of the Vanadium, the profile for vanadium is flatter relative to time as even the freshest part of the inventory contains a substantial amount of Vanadium.
Thus the nature of ECAT's is such that every unit has different ECAT properties. In general, units processing light feedstocks result in ECAT with low Ni, Vanadium and other contaminants. Also because of the low coking tendency of the feedstock combined with low ECAT dehydrogenation activity (e.g. coke and H2 yield), low metal ECAT are generally very active as they have retained a substantial percentage of the fresh surface area. On the contrary, units processing heavy or highly contaminated feedstocks with high levels of Ni, V and other contaminants result in ECAT with high levels of these undesired metals. These metals result in severe catalyst deactivation and high dehydrogenation activity (e.g. coke and H2 yield) which make the catalyst make up requirements much higher. Because of the heavy feedstock's higher coking tendency combined with the fresh catalyst activity and coke selectivity, the fresh CAR for these heavy feedstock FCC units can become limited by the coke yield. In this case, addition of a catalyst with low activity (e.g. ECAT) can be used to improve the coke selectivity of the system. In some cases, operators use this type of ECAT “flushing” to maintain catalysts cost within a budget. In practice, the ECAT that is withdrawn from units that process feedstocks with low metals (“high quality, low metals, high activity ECAT”) is injected into units that process feedstocks with higher metals in order to control metal levels on ECAT or to manage the proper activity and selectivity levels. These heavy feedstock units result in ECATS with much lower activity and higher metals than the low metal operation. The consequence of this addition is that metals in the high metal operation are “diluted” relative to an operation in which only fresh catalyst is added. The optimal conditions are unit dependent and in some cases it is not economical to add the ECAT from certain units to other units that process feed with higher metals.
Thus, ECAT with relatively low metals has economic value for some operations. Although exact values may vary, ECAT with Nickel and Vanadium levels less than 1500 ppm each and with total levels (Ni+V) preferably less than 2500 ppm are considered valuable. ECAT with higher metals is usually sent to cement kilns and or sent for disposal, usually at a substantial cost to the operation. In general high quality ECAT is sold for reuse at $600-1000/MT depending on ECAT composition and market conditions. Refiners sell high quality ECAT to ECAT brokers at prices varying from $100-$600/MT. When ECAT is sent for disposal or use in cement kilns, ECAT has usually zero or negative value to pay for associated transportation and disposal costs. Blending of low value ECAT with high quality ECAT is practiced to meet a set of specifications.
A good illustration of vanadium and nickel distribution in ECAT is found in the paper of R. Pimenta, A. R, Quinones, and P. Imhof titled “FCC Testing Philosophy” published privately by Akzo Nobel International Symposium, Noordwijk, The Netherlands, 1998. showing analyses of equilibrium catalyst fractionated by using liquids of different densities, without attempting to segregate even quartiles by weight. Table 2 from that paper is reproduced in part below as Table I:
TABLE IFractionECATLightestMed-LightMed-heavyHeavyWt % of Sample100%21%22%20%37%Surf Area, Mg2/g138183166141106MiPore Area, Mg2/g11213812810982Nickel, ppm2,1787151,0071,7773,721Vanadium, ppm5,0803,5114,2715,0316,264Ni/V, wt/wt0.42870.20360.23580.35320.5940
As illustrated above and further explained in the above cited paper and elsewhere in the literature, the nickel and vanadium contents are important determinants of the weight ranking. Particles having higher V and Ni contents are older and less active than the others. The mobility of the vanadium is reflected in the Ni/V ratio, and clearly trends upward as the particles become older.
In the prior art, researchers have focused on and patented methodologies to recover the low metal part of the ECAT. The so called MAGNACAT process used magnetic separation. Others have tried to selectively remove the metal contaminants.
In U.S. Pat. No. 5,393,412, Hettinger et at employ an attriting process to generate smaller particulates while also removing undesired metals on the surfaces of the catalyst; they can combine this approach with certain magnetic and size classifier techniques. Goolsby et at in U.S. Pat. No. 7,431,826 combine a magnetic separation step with gravity separation; the magnetic sepration step may also be accompanied by the use of a filter screen to remove larger particles.
Air cyclone separations are used to adjust the particle size of fresh catalysts to make them conform with FCC unit requirements to avoid excessive losses. This application is clearly different than the present invention in that it is utilized only on homogenous fresh catalyst systems.
Magnetic systems are expensive to install, to operate, and to maintain. The art is still in need of a reliable, economic method of recycling as much ECAT as is reasonably possible.